First of all, you cannot "became a citizen through an N-600 application". The N-600 is the form to apply for a Certificate of Citizenship, for which the prerequisite is that you are already a U.S. citizen. The certificate is simply a proof. Applying for it, or not applying for it, has no effect on your U.S. citizenship. You became a U.S. citizen automatically (and involuntarily) by law when your father naturalized, and you were a permanent resident under 18 residing with him.
Every country's nationality law is independent, and to determine whether you have a country's nationality, you must look to that country's laws. Canadian nationality law does not care about other nationalities. So the question is with South Africa.
For South Africa, you automatically lose South African nationality on acquiring another nationality only if you were at least 18 when you acquired it, acquired it voluntarily, and if you did not get permission to retain it. This doesn't apply to you because you acquired U.S. nationality (and Canadian too, if you weren't born with it) before 18 (also it was not voluntary). You can read the South African Citizenship Act of 1995, in particular, article 6, which says:
(1) Subject to the provisions of subsection (2), a South African citizen shall cease to be a South African citizen if-
(a) he or she, whilst not being a minor, by some voluntary and formal act other than marriage, acquires the citizenship or
nationality of a country other than the Republic; or
(b) he or she in terms of the laws of any other country also has the citizenship or nationality of that country, and serves in the
armed forces of such country while that country is at war with the
Republic.
(2) Any person referred to in subsection (1) may, prior to his or her
loss of South African citizenship in terms of this section, apply to
the Minister to retain his or her South African citizenship, and the
Minister may, if he or she deems it fit, order such retention.
Renunciation of citizenship
